When a person, who may or may not be a douchebag, commits several obnoxious actions that would make them seem as if they were on a douchebag rampage.

For instance, this person could all of a sudden pull out a pair of aviators, twist their cap backwards, chug a beer and attempt to crush the can on their forehead, then pump up some really loud bass beats and begin to fist pump.
Also, these people could simply be considered assholes and/or attention whores, who will go on a rampage by verbally annoying everyone around them if they are not being paid attention to, saying stupid things like "Yeah bro", "Yo, none of these broads are good enough for me", or "That's why everybody loves me man".

This makes everyone around this person want to inflict serious bodily harm to them, especially to their genitals, so that they cannot foul the earth with another little shit like themselves.

Person 1:"Oh my God, did you see that guy start fist-pumping at the bar after he chugged that beer? He almost knocked his stupid visor off his head-and how can he even see? He's wearing sunglasses indoors!"
Person 2:"Yeah I saw him..he's commiting an act of Rampant Douchebaggery right now. Just ignore him-these attention whores will calm down when they see everybody hates them."